{"id":7609,"date":"2026-10-27T01:14:36","date_gmt":"2026-10-27T01:14:36","guid":{"rendered":"https:\/\/uplastech.com\/pom-acetal-injection-molding-guide\/"},"modified":"2026-10-27T01:14:36","modified_gmt":"2026-10-27T01:14:36","slug":"pom-acetal-injection-molding-guide","status":"publish","type":"post","link":"https:\/\/uplastech.com\/tr\/pom-acetal-injection-molding-guide\/","title":{"rendered":"POM (Acetal) Injection Molding: Thermal Stability and Tooling"},"content":{"rendered":"<style>\n.upl-blog{font-size:17px;line-height:1.78;color:#1f2937}\n.upl-blog p{margin:0 0 18px}\n.upl-blog h2{font-size:26px;line-height:1.3;color:#0b2b33;margin:46px 0 14px;font-weight:700;letter-spacing:-.015em}\n.upl-blog h3{font-size:19.5px;color:#0d3b47;margin:28px 0 10px;font-weight:700}\n.upl-blog ul,.upl-blog ol{margin:0 0 20px;padding-left:22px}\n.upl-blog li{margin:0 0 9px}\n.upl-blog figure{margin:0 0 34px}\n.upl-blog figure img{width:100%;height:auto;display:block;border-radius:10px}\n.upl-blog figcaption{font-size:13px;color:#6b7280;margin-top:10px;line-height:1.5}\n.upl-blog .toc{background:none;border:0;border-radius:0;padding:0;margin:0 0 34px}\n.upl-blog .toc .lwptoc{margin:0}\n.upl-blog .toc h4{margin:0 0 10px;font-size:13px;letter-spacing:.1em;text-transform:uppercase;color:#0d5563}\n.upl-blog .toc ol{margin:0;padding-left:18px;font-size:14.5px}\n.upl-blog .toc li{margin-bottom:6px}\n.upl-blog table{width:100%;border-collapse:collapse;font-size:14.5px;margin:0 0 10px}\n.upl-blog th{background:#0d3b47;color:#fff;text-align:left;padding:11px 13px;font-weight:600;font-size:13.5px;border:0}\n.upl-blog td{padding:11px 13px;border:0;border-bottom:1px solid #e8ebed;vertical-align:top}\n.upl-blog tbody tr:nth-child(even){background:#fafbfc}\n.upl-blog .tablecap{font-size:12.5px;color:#6b7280;margin:0 0 30px}\n.upl-blog .myth{background:#fff8ef;border:1px solid #f6ddbd;border-radius:8px;padding:16px 20px;margin:0 0 26px;font-size:15.5px}\n.upl-blog .myth b{color:#a85c10}\n.upl-blog .callout{background:#f2f8f9;border-radius:8px;padding:18px 22px;margin:0 0 26px;font-size:15.5px;border-left:3px solid #14707f}\n.upl-blog .faq h3{border-top:1px solid #eceff1;padding-top:22px;margin-top:0;font-size:18px}\n.upl-blog .faq h3:first-of-type{border-top:0;padding-top:0}\n.upl-blog .cta{background:linear-gradient(135deg,#0b2b33,#0d5563);color:#fff;border-radius:12px;padding:34px;margin:48px 0 30px}\n.upl-blog .cta h2,.upl-blog .cta .cta-h{color:#fff;margin:0 0 10px;font-size:24px;font-weight:700;line-height:1.3}\n.upl-blog .cta p{color:#c9dee3;margin:0 0 20px;font-size:16px}\n.upl-blog .btn{display:inline-block;background:#e07a1f;color:#fff;text-decoration:none;font-weight:700;padding:13px 26px;border-radius:8px;font-size:15.5px}\n.upl-blog .rel{border-top:1px solid #e8ebed;margin-top:40px;padding-top:26px}\n.upl-blog .rel h4{font-size:13px;letter-spacing:.1em;text-transform:uppercase;color:#6b7280;margin:0 0 14px}\n.upl-blog .rel ul{list-style:none;padding:0;margin:0}\n.upl-blog .rel li{padding:11px 0;border-bottom:1px dashed #e8ebed}\n.upl-blog .rel a{text-decoration:none;font-weight:600;font-size:15.5px}\n@media(max-width:640px){.upl-blog h2{font-size:22px}.upl-blog table{font-size:13px}.upl-blog .cta{padding:24px}}\n\/* \u5355\u7bc7\u9875\u9690\u85cf WhatsApp \u81ea\u52a8\u5c55\u5f00\u7684\u95ee\u5019\u5361\uff0c\u53ea\u4fdd\u7559\u5706\u5f62\u6c14\u6ce1\uff0c\u907f\u514d\u906e\u6321\u4fa7\u680f\u8be2\u76d8\u63d0\u4ea4\u6309\u94ae *\/\nbody.single-post .ht_ctc_chat_greetings_box{display:none !important}\n<\/style>\n<div class=\"upl-blog\">\n<figure>\n<img fetchpriority=\"high\" decoding=\"async\" src=\"https:\/\/uplastech.com\/wp-content\/uploads\/2026\/10\/im-pom-part.jpg\" alt=\"White and black POM acetal injection moulded gears, bushings and clips arranged on a workshop bench\" width=\"1536\" height=\"909\"><figcaption>POM gears, bushings and clips. Every one of these is a part where dimensional stability, low friction and machinability matter more than strength \u2014 which is the case where acetal beats nylon.<\/figcaption><\/figure>\n<div class=\"toc\"><\/div>\n<h2 id=\"why\">What POM is for<\/h2>\n<p>Polyoxymethylene \u2014 acetal, POM \u2014 is a highly crystalline engineering plastic with an unusual combination: high stiffness, low friction, excellent fatigue resistance, very good machinability, and dimensional stability that nylon cannot match because POM absorbs almost no water.<\/p>\n<p>That last property is why POM wins so many precision parts. A POM gear holds its dimensions from the day it is moulded to the day it is scrapped, in any humidity. A nylon gear does not. For anything that must fit, run quietly and stay in tolerance for years, POM is usually the right answer.<\/p>\n<div class=\"callout\">\n<p><strong>The two numbers that govern POM processing:<\/strong> erime s\u0131cakl\u0131\u011f\u0131 <strong>190 \u2013 210 \u00b0C<\/strong> and residence time <strong>under 6 minutes<\/strong>. POM is thermally unstable outside a narrow band \u2014 too cold and it does not melt, too hot or too long and it unzips into formaldehyde, which is both a health hazard and a self-accelerating decomposition. There is no forgiving zone.<\/p>\n<\/div>\n<h2 id=\"grades\">Grades<\/h2>\n<table>\n<thead>\n<tr>\n<th>Grade<\/th>\n<th>Key property<\/th>\n<th>Used for<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Homopolymer (POM-H)<\/td>\n<td>Higher stiffness, strength and creep resistance<\/td>\n<td>Gears, precision parts, load-bearing<\/td>\n<\/tr>\n<tr>\n<td>Copolymer (POM-C)<\/td>\n<td>Better thermal and chemical stability, easier processing<\/td>\n<td>General parts, hot-water contact, food<\/td>\n<\/tr>\n<tr>\n<td>Impact modified<\/td>\n<td>Notched Izod 80 \u2013 120 J\/m<\/td>\n<td>Clips, latches, snap fits<\/td>\n<\/tr>\n<tr>\n<td>Glass filled, 20 \u2013 30 %<\/td>\n<td>Modulus 6 \u2013 9 GPa, low creep<\/td>\n<td>Structural brackets<\/td>\n<\/tr>\n<tr>\n<td>Low friction \/ PTFE filled<\/td>\n<td>\u00b5 down to 0.08<\/td>\n<td>Bearings, sliding parts<\/td>\n<\/tr>\n<tr>\n<td>Anti-static \/ conductive<\/td>\n<td>Surface resistivity 10\u00b3 \u2013 10\u2076 \u03a9<\/td>\n<td>Electronics handling, fuel systems<\/td>\n<\/tr>\n<tr>\n<td>UV stabilised<\/td>\n<td>Carbon black or HALS<\/td>\n<td>Outdoor parts<\/td>\n<\/tr>\n<tr>\n<td>Food contact \/ medical<\/td>\n<td>Compliant grades<\/td>\n<td>Food equipment, drug delivery<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 1 \u2014 POM grade families. The homopolymer \/ copolymer choice is the first decision.<\/p>\n<p>The homopolymer versus copolymer distinction matters more than it does for most materials. POM-H is about 10 \u2013 15 % stiffer and stronger, and has better creep resistance. POM-C is more resistant to thermal degradation, more resistant to hot water and weak acids, and has a wider processing window. For a part that will be moulded in a difficult tool or run on an old machine, POM-C is the safer choice.<\/p>\n<h2 id=\"properties\">\u00d6zellikler<\/h2>\n<table>\n<thead>\n<tr>\n<th>M\u00fclkiyet<\/th>\n<th>POM-H<\/th>\n<th>POM-C<\/th>\n<th>30 % GF<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Density, g\/cm\u00b3<\/td>\n<td>1.42 \u2013 1.43<\/td>\n<td>1.40 \u2013 1.41<\/td>\n<td>1.55 \u2013 1.60<\/td>\n<\/tr>\n<tr>\n<td>Tensile strength, MPa<\/td>\n<td>68 \u2013 75<\/td>\n<td>60 \u2013 65<\/td>\n<td>110 \u2013 135<\/td>\n<\/tr>\n<tr>\n<td>Flexural modulus, GPa<\/td>\n<td>2.8 \u2013 3.2<\/td>\n<td>2.5 \u2013 2.8<\/td>\n<td>7.0 \u2013 9.0<\/td>\n<\/tr>\n<tr>\n<td>Notched Izod, J\/m<\/td>\n<td>60 \u2013 90<\/td>\n<td>60 \u2013 80<\/td>\n<td>50 \u2013 70<\/td>\n<\/tr>\n<tr>\n<td>HDT at 1.82 MPa<\/td>\n<td>95 \u2013 100 \u00b0C<\/td>\n<td>85 \u2013 95 \u00b0C<\/td>\n<td>155 \u2013 165 \u00b0C<\/td>\n<\/tr>\n<tr>\n<td>Shrinkage, %<\/td>\n<td>1.8 \u2013 2.5<\/td>\n<td>1.8 \u2013 2.2<\/td>\n<td>0.3 \u2013 0.6<\/td>\n<\/tr>\n<tr>\n<td>Water absorption, 24 h<\/td>\n<td>0.2 %<\/td>\n<td>0.2 %<\/td>\n<td>0.15 %<\/td>\n<\/tr>\n<tr>\n<td>Water absorption, equilibrium<\/td>\n<td>0.5 \u2013 0.8 %<\/td>\n<td>0.5 \u2013 0.8 %<\/td>\n<td>0.4 %<\/td>\n<\/tr>\n<tr>\n<td>Coefficient of friction on steel<\/td>\n<td>0.15 \u2013 0.25<\/td>\n<td>0.15 \u2013 0.25<\/td>\n<td>0.25 \u2013 0.35<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 2 \u2014 POM properties. The equilibrium water absorption of 0.5 \u2013 0.8 % is 3 \u2013 4\u00d7 lower than nylon.<\/p>\n<p>That last row is the commercial argument for POM over nylon in precision parts. Nylon grows 0.3 \u2013 0.6 % in service; POM grows 0.1 \u2013 0.2 %. On a 50 mm part that is the difference between 0.25 mm of movement and 0.06 mm \u2014 the difference between a tolerance that holds and one that does not.<\/p>\n<h2 id=\"stability\">Thermal stability: the failure mode that matters<\/h2>\n<p>POM decomposes by unzipping \u2014 the polymer chain depolymerises from the end, releasing formaldehyde, and the formaldehyde catalyses further decomposition. Once it starts, it accelerates. A barrel left hot with POM in it over a weekend can be destroyed, and the gas is flammable and toxic.<\/p>\n<table>\n<thead>\n<tr>\n<th>Condition<\/th>\n<th>Result<\/th>\n<th>Prevention<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Melt above 230 \u00b0C<\/td>\n<td>Rapid decomposition, formaldehyde smell<\/td>\n<td>Hard limit at 220 \u00b0C; 210 \u00b0C preferred<\/td>\n<\/tr>\n<tr>\n<td>Residence time above 10 min<\/td>\n<td>Yellowing, then bubbling, then decomposition<\/td>\n<td>Size the shot to 60 \u2013 80 % of barrel capacity<\/td>\n<\/tr>\n<tr>\n<td>Machine stopped hot<\/td>\n<td>Decomposition within 20 \u2013 30 min<\/td>\n<td>Purge with PE or PP before stopping; or cool the barrel<\/td>\n<\/tr>\n<tr>\n<td>Contamination with PVC or flame retardant<\/td>\n<td>Acid or halogen catalyses decomposition<\/td>\n<td>Dedicated barrel, or a thorough purge<\/td>\n<\/tr>\n<tr>\n<td>Hot-runner dead spot<\/td>\n<td>Slow decomposition over days<\/td>\n<td>No dead spots; smooth, streamlined flow paths<\/td>\n<\/tr>\n<tr>\n<td>Regrind with degraded material<\/td>\n<td>Seeds decomposition in the next shot<\/td>\n<td>Discard yellowed or bubbled parts; do not regrind them<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 3 \u2014 POM stability failures. Every one of these is avoidable with discipline.<\/p>\n<p>The practical rules that come out of this table:<\/p>\n<ul>\n<li><strong>Size the machine to the tool.<\/strong> A shot that uses less than 40 % of the barrel capacity leaves material in the barrel for too long. This is the most common cause of POM problems in job shops.<\/li>\n<li><strong>Purge before every stop.<\/strong> Natural PE or PP is the standard purge; there are also commercial POM purge compounds.<\/li>\n<li><strong>Never let the barrel sit hot and idle for more than 15 minutes.<\/strong> If a run pauses, either keep cycling or purge and shut down.<\/li>\n<li><strong>Ventilate.<\/strong> Formaldehyde is a respiratory irritant and a suspected carcinogen. Extraction at the nozzle and at the mould is normal practice.<\/li>\n<\/ul>\n<div class=\"myth\">\n<p><b>Myth: \"POM is easy \u2014 it just runs.\"<\/b> POM runs beautifully inside its window and destroys itself outside it. The shops that run POM well are the ones with a written rule about shot size and a purge discipline, not the ones with the newest machine.<\/p>\n<\/div>\n<h2 id=\"processing\">Processing window<\/h2>\n<table>\n<thead>\n<tr>\n<th>Parametre<\/th>\n<th>Typical setting<\/th>\n<th>Notlar<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Erime s\u0131cakl\u0131\u011f\u0131<\/td>\n<td>190 \u2013 210 \u00b0C<\/td>\n<td>Never above 220 \u00b0C<\/td>\n<\/tr>\n<tr>\n<td>Barrel profile<\/td>\n<td>170 \/ 185 \/ 195 \/ 200 \u00b0C<\/td>\n<td>Flat profile; avoid hot spots<\/td>\n<\/tr>\n<tr>\n<td>Nozzle<\/td>\n<td>195 \u2013 205 \u00b0C<\/td>\n<td>Use a large, free-flow nozzle<\/td>\n<\/tr>\n<tr>\n<td>Mould temperature<\/td>\n<td>80 \u2013 110 \u00b0C<\/td>\n<td>Hot mould = high crystallinity = strength and wear<\/td>\n<\/tr>\n<tr>\n<td>Injection pressure<\/td>\n<td>70 \u2013 120 MPa<\/td>\n<td>High, because POM is stiff when molten<\/td>\n<\/tr>\n<tr>\n<td>Injection speed<\/td>\n<td>Moderate to fast<\/td>\n<td>Fast fill avoids the cold-slug problem<\/td>\n<\/tr>\n<tr>\n<td>Hold pressure<\/td>\n<td>60 \u2013 80 % of injection<\/td>\n<td>POM needs firm packing; shrinkage is high<\/td>\n<\/tr>\n<tr>\n<td>Back pressure<\/td>\n<td>0.5 \u2013 1.0 MPa<\/td>\n<td><\/td>\n<\/tr>\n<tr>\n<td>Vida h\u0131z\u0131<\/td>\n<td>50 \u2013 90 rpm<\/td>\n<td><\/td>\n<\/tr>\n<tr>\n<td>Drying<\/td>\n<td>2 h at 80 \u2013 90 \u00b0C if stored humid<\/td>\n<td>Not normally needed; bags are fine<\/td>\n<\/tr>\n<tr>\n<td>Regrind<\/td>\n<td>Up to 20 %<\/td>\n<td>Same grade only; never degraded material<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 4 \u2014 POM processing. The 220 \u00b0C ceiling is not a suggestion.<\/p>\n<p>Mould temperature is the quality lever. At 60 \u00b0C a POM part has a thin crystalline skin over a less crystalline core, and it will wear faster and creep more. At 100 \u00b0C the part is uniformly crystalline, dimensionally stable and has the wear resistance the datasheet promises. The cycle is longer \u2014 25 \u2013 35 % longer than a cold mould \u2014 but for a gear or a bearing, it is the difference between a part that lasts and one that does not.<\/p>\n<h2 id=\"design\">Design rules<\/h2>\n<table>\n<thead>\n<tr>\n<th>\u00d6zellik<\/th>\n<th>Rule<\/th>\n<th>Why POM is different<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Wall thickness<\/td>\n<td>1.0 \u2013 3.0 mm; uniform within 20 %<\/td>\n<td>High shrinkage makes thick sections sink and void<\/td>\n<\/tr>\n<tr>\n<td>Draft<\/td>\n<td>0.5 \u2013 1\u00b0<\/td>\n<td>POM is slippery and releases well<\/td>\n<\/tr>\n<tr>\n<td>Internal radius<\/td>\n<td>\u2265 0.5 \u00d7 wall<\/td>\n<td>Notch sensitive under impact<\/td>\n<\/tr>\n<tr>\n<td>Ribs<\/td>\n<td>0.5 \u00d7 wall, spacing \u2265 2 \u00d7 wall<\/td>\n<td>Thick ribs sink visibly<\/td>\n<\/tr>\n<tr>\n<td>Bosses<\/td>\n<td>Wall 0.6 \u00d7 nominal<\/td>\n<td><\/td>\n<\/tr>\n<tr>\n<td>Snap fits<\/td>\n<td>Strain \u2264 2 % for repeated use, 4 % single assembly<\/td>\n<td>POM is springy but fatigues<\/td>\n<\/tr>\n<tr>\n<td>Living hinge<\/td>\n<td>Possible in POM-C, but PP is better<\/td>\n<td>POM hinges work but have shorter life<\/td>\n<\/tr>\n<tr>\n<td>Gate<\/td>\n<td>Large; direct or edge gate preferred<\/td>\n<td>POM is shear sensitive<\/td>\n<\/tr>\n<tr>\n<td>Shrinkage allowance<\/td>\n<td>2.0 % nominal, confirm by trial<\/td>\n<td>Higher than most; the tool must be cut for it<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 5 \u2014 POM design rules. The 2 % shrinkage is the number toolmakers must not guess at.<\/p>\n<h2 id=\"tooling\">Tooling: steel, gates and hot runners<\/h2>\n<table>\n<thead>\n<tr>\n<th>Item<\/th>\n<th>Tavsiye<\/th>\n<th>Reason<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Mould steel<\/td>\n<td>Hardened stainless (420 or equivalent, 48 \u2013 52 HRC)<\/td>\n<td>Formaldehyde attacks plain steel; corrosion causes sticking and marks<\/td>\n<\/tr>\n<tr>\n<td>Core and cavity finish<\/td>\n<td>SPI B-1 or better<\/td>\n<td>POM grips a rough surface<\/td>\n<\/tr>\n<tr>\n<td>Gates<\/td>\n<td>Large, with a short land<\/td>\n<td>Reduces shear heating<\/td>\n<\/tr>\n<tr>\n<td>Hot runner<\/td>\n<td>Only with streamlined, dead-spot-free tips<\/td>\n<td>A dead spot decomposes slowly and ruins every shot<\/td>\n<\/tr>\n<tr>\n<td>Hot-runner temperature<\/td>\n<td>190 \u2013 200 \u00b0C, tightly controlled<\/td>\n<td>\u00b12 \u00b0C control is required<\/td>\n<\/tr>\n<tr>\n<td>Ejection<\/td>\n<td>Generous ejector area<\/td>\n<td>POM is stiff; thin ejectors mark the part<\/td>\n<\/tr>\n<tr>\n<td>Venting<\/td>\n<td>0.02 \u2013 0.03 mm depth, generous<\/td>\n<td>Trapped gas burns and the gas is corrosive<\/td>\n<\/tr>\n<tr>\n<td>So\u011futma<\/td>\n<td>Uniform, both halves<\/td>\n<td>Asymmetric cooling warps a high-shrinkage material badly<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 6 \u2014 POM tooling. Stainless steel is not a luxury \u2014 it is a maintenance requirement.<\/p>\n<h2 id=\"machining\">POM as a machined material<\/h2>\n<p>POM is one of the best engineering plastics to machine: it produces a continuous chip, holds tolerance, takes a fine finish, and does not need coolant for most operations. That makes it the standard choice for prototypes and low-volume precision parts before a tool exists.<\/p>\n<table>\n<thead>\n<tr>\n<th>Operation<\/th>\n<th>Notlar<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>D\u00f6n\u00fc\u015f<\/td>\n<td>Sharp, high-rake tools; positive rake; avoid rubbing<\/td>\n<\/tr>\n<tr>\n<td>Frezeleme<\/td>\n<td>Climb milling preferred; sharp cutters; air blast rather than flood coolant<\/td>\n<\/tr>\n<tr>\n<td>Sondaj<\/td>\n<td>Slow helix, sharp point; clear chips frequently \u2014 POM grabs a drill<\/td>\n<\/tr>\n<tr>\n<td>Tapping<\/td>\n<td>Oversize the tap 0.05 mm; POM springs back<\/td>\n<\/tr>\n<tr>\n<td>Annealing before machining<\/td>\n<td>2 h at 140 \u2013 150 \u00b0C for extruded stock, to relieve stress<\/td>\n<\/tr>\n<tr>\n<td>Coolant<\/td>\n<td>Water-soluble is fine; avoid neat oils that swell the surface<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 7 \u2014 Machining POM. The tap oversize note saves a lot of broken taps.<\/p>\n<h2 id=\"defects\">Defects and causes<\/h2>\n<table>\n<thead>\n<tr>\n<th>Kusur<\/th>\n<th>Neden<\/th>\n<th>Fix<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Yellow or brown parts<\/td>\n<td>Over-heating or long residence<\/td>\n<td>Lower melt; reduce shot-to-barrel ratio; purge<\/td>\n<\/tr>\n<tr>\n<td>Bubbles \/ voids<\/td>\n<td>Decomposition gas, or shrinkage voids<\/td>\n<td>Check residence time; increase hold pressure<\/td>\n<\/tr>\n<tr>\n<td>Formaldehyde smell<\/td>\n<td>Decomposition starting<\/td>\n<td>Purge immediately; find the hot spot<\/td>\n<\/tr>\n<tr>\n<td>Black specks<\/td>\n<td>Carbonised material in the barrel or hot runner<\/td>\n<td>Full purge with PE; strip and clean the hot runner<\/td>\n<\/tr>\n<tr>\n<td>Sink marks<\/td>\n<td>Thick sections, low hold pressure<\/td>\n<td>Uniform walls; increase hold<\/td>\n<\/tr>\n<tr>\n<td>\u00c7arp\u0131kl\u0131k<\/td>\n<td>Asymmetric cooling, high shrinkage<\/td>\n<td>Balance mould temperature; uniform walls<\/td>\n<\/tr>\n<tr>\n<td>Poor weld line strength<\/td>\n<td>Low mould temperature<\/td>\n<td>Raise mould to 90 \u2013 100 \u00b0C<\/td>\n<\/tr>\n<tr>\n<td>Sticking \/ drag marks<\/td>\n<td>Over-packing, rough tool, insufficient draft<\/td>\n<td>Reduce hold; polish the tool; add draft<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 8 \u2014 POM defects. Any smell of formaldehyde means stop and purge.<\/p>\n<h2 id=\"applications\">Where POM wins over nylon<\/h2>\n<table>\n<thead>\n<tr>\n<th>Uygulama<\/th>\n<th>Why POM<\/th>\n<th>Why not nylon<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Precision gears<\/td>\n<td>Holds tolerance in humidity<\/td>\n<td>Nylon grows 0.3 \u2013 0.6 %<\/td>\n<\/tr>\n<tr>\n<td>Bushes and bearings<\/td>\n<td>Low friction, low wear, stable<\/td>\n<td>Nylon swells and loses clearance<\/td>\n<\/tr>\n<tr>\n<td>Fuel system components<\/td>\n<td>Resistant to petrol and diesel<\/td>\n<td>Nylon is also good but absorbs alcohol<\/td>\n<\/tr>\n<tr>\n<td>Clips and latches<\/td>\n<td>Springy, fatigue resistant<\/td>\n<td>Nylon is tougher but creeps<\/td>\n<\/tr>\n<tr>\n<td>Plumbing components<\/td>\n<td>Resistant to hot water (copolymer)<\/td>\n<td>Nylon hydrolyses in hot water<\/td>\n<\/tr>\n<tr>\n<td>Food equipment parts<\/td>\n<td>Approved grades, machinable, no odour<\/td>\n<td>Nylon can impart taste<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 9 \u2014 POM versus nylon. The hot-water row is decisive: nylon degrades in continuous hot water, POM-C does not.<\/p>\n<h2 id=\"mouldtemp\">Mould temperature and the cycle-time trade<\/h2>\n<p>POM is the clearest case in injection moulding where a longer cycle buys a better part. The mould temperature controls crystallinity, and crystallinity controls wear resistance, creep and dimensional stability.<\/p>\n<table>\n<thead>\n<tr>\n<th>Mould temperature<\/th>\n<th>Crystallinity<\/th>\n<th>Cycle time index<\/th>\n<th>Wear resistance<\/th>\n<th>Verdict<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>50 \u00b0C<\/td>\n<td>55 \u2013 60 %<\/td>\n<td>100<\/td>\n<td>Zay\u0131f<\/td>\n<td>Cosmetic parts only<\/td>\n<\/tr>\n<tr>\n<td>70 \u00b0C<\/td>\n<td>65 \u2013 70 %<\/td>\n<td>115<\/td>\n<td>Adil<\/td>\n<td>General parts<\/td>\n<\/tr>\n<tr>\n<td>90 \u00b0C<\/td>\n<td>72 \u2013 78 %<\/td>\n<td>132<\/td>\n<td>\u0130yi<\/td>\n<td>Gears, bearings<\/td>\n<\/tr>\n<tr>\n<td>110 \u00b0C<\/td>\n<td>78 \u2013 82 %<\/td>\n<td>150<\/td>\n<td>Best<\/td>\n<td>Precision wear parts<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 9 \u2014 Mould temperature versus crystallinity for POM. A 50 % cycle increase buys roughly double the wear life.<\/p>\n<p>Whether that trade is worth it depends entirely on the part. For a cosmetic clip, a cold mould is correct \u2014 the part is not loaded and the cycle matters. For a gear running at 3,000 rpm in a power tool, a 90 \u2013 100 \u00b0C mould is the difference between a gearbox that lasts five years and one that whines and fails in eighteen months. Both parts might come off the same machine on the same day, and they need different moulds.<\/p>\n<h2 id=\"tolerances\">What tolerance POM can actually hold<\/h2>\n<table>\n<thead>\n<tr>\n<th>Dimension type<\/th>\n<th>Achievable tolerance<\/th>\n<th>Notlar<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Overall dimension, &lt; 30 mm<\/td>\n<td>&plusmn;0.05 mm<\/td>\n<td>With a stable process and a corrected tool<\/td>\n<\/tr>\n<tr>\n<td>Overall dimension, 30 \u2013 80 mm<\/td>\n<td>&plusmn;0.08 mm<\/td>\n<td><\/td>\n<\/tr>\n<tr>\n<td>Overall dimension, &gt; 80 mm<\/td>\n<td>&plusmn;0.15 mm<\/td>\n<td>Shrinkage variation dominates<\/td>\n<\/tr>\n<tr>\n<td>Wall thickness<\/td>\n<td>&plusmn;0.08 mm<\/td>\n<td>Worse than overall dimension<\/td>\n<\/tr>\n<tr>\n<td>Hole diameter, moulded pin<\/td>\n<td>&plusmn;0.05 mm<\/td>\n<td>POM holds holes well<\/td>\n<\/tr>\n<tr>\n<td>Centre distance, two features<\/td>\n<td>&plusmn;0.05 mm<\/td>\n<td>Set by the tool, not the process<\/td>\n<\/tr>\n<tr>\n<td>Gear tooth profile<\/td>\n<td>DIN 9 \u2013 10<\/td>\n<td>AGMA 8 \u2013 9 equivalent<\/td>\n<\/tr>\n<tr>\n<td>Flatness on a 50 mm face<\/td>\n<td>0.1 \u2013 0.2 mm<\/td>\n<td>Depends on gate position<\/td>\n<\/tr>\n<tr>\n<td>Straightness on a long part<\/td>\n<td>2 \u2013 4 mm\/m<\/td>\n<td>Cooling asymmetry dominates<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 10 \u2014 Realistic POM tolerances. These are production figures, not best-case.<\/p>\n<p>The reason POM holds tolerance better than most semi-crystalline materials is that it does not absorb water \u2014 the dimension you measure a day after moulding is the dimension it will have next year. That stability is worth more than the absolute tolerance figure, because it means a part that passes at incoming inspection still passes at assembly, six months later, in a humid warehouse.<\/p>\n<h2 id=\"versus\">POM versus other wear materials<\/h2>\n<table>\n<thead>\n<tr>\n<th>M\u00fclkiyet<\/th>\n<th>POM<\/th>\n<th>PA66<\/th>\n<th>PBT<\/th>\n<th>PPS<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Coefficient of friction<\/td>\n<td>0.15 \u2013 0.25<\/td>\n<td>0.20 \u2013 0.35<\/td>\n<td>0.35 \u2013 0.45<\/td>\n<td>0.30 \u2013 0.40<\/td>\n<\/tr>\n<tr>\n<td>Wear rate (unlubricated)<\/td>\n<td>D\u00fc\u015f\u00fck<\/td>\n<td>Low\u2013moderate<\/td>\n<td>Orta d\u00fczeyde<\/td>\n<td>D\u00fc\u015f\u00fck<\/td>\n<\/tr>\n<tr>\n<td>Dimensional stability in humidity<\/td>\n<td>M\u00fckemmel<\/td>\n<td>Zay\u0131f<\/td>\n<td>M\u00fckemmel<\/td>\n<td>M\u00fckemmel<\/td>\n<\/tr>\n<tr>\n<td>Impact strength<\/td>\n<td>Orta d\u00fczeyde<\/td>\n<td>Y\u00fcksek<\/td>\n<td>D\u00fc\u015f\u00fck<\/td>\n<td>D\u00fc\u015f\u00fck<\/td>\n<\/tr>\n<tr>\n<td>Continuous temperature<\/td>\n<td>90 \u2013 105 \u00b0C<\/td>\n<td>100 \u2013 120 \u00b0C<\/td>\n<td>120 \u2013 140 \u00b0C<\/td>\n<td>200 \u2013 220 \u00b0C<\/td>\n<\/tr>\n<tr>\n<td>Machinability<\/td>\n<td>M\u00fckemmel<\/td>\n<td>Adil<\/td>\n<td>Adil<\/td>\n<td>Zay\u0131f<\/td>\n<\/tr>\n<tr>\n<td>Relative cost<\/td>\n<td>1\u00d7<\/td>\n<td>1.1\u00d7<\/td>\n<td>1.3\u00d7<\/td>\n<td>4 \u2013 6\u00d7<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 11 \u2014 Wear material comparison. POM wins on friction plus stability plus machinability, which is a combination nothing else matches.<\/p>\n<p>The temperature column is where POM loses. Above 100 \u00b0C continuous, POM softens and creeps, and a gearbox running at 110 \u00b0C will fail even though the material was correct at room temperature. That is the point at which PBT or PPS becomes necessary \u2014 and the point at which the part stops being machinable and the tooling cost goes up.<\/p>\n<h2 id=\"faq\">Frequently asked questions<\/h2>\n<div class=\"faq\">\n<h3>What is the maximum melt temperature for POM?<\/h3>\n<p>220 \u00b0C as an absolute ceiling, 210 \u00b0C in normal running, 200 \u00b0C for copolymers with a long residence time. Above 230 \u00b0C decomposition is rapid and self-accelerating.<\/p>\n<h3>Why does my POM part smell of formaldehyde?<\/h3>\n<p>Because it is decomposing. Either the melt is too hot, the residence time is too long, or there is a dead spot in the hot runner or nozzle. Purge with natural PE immediately, then find the cause. Do not keep running and hope it clears.<\/p>\n<h3>Can POM be run in the same barrel as PVC?<\/h3>\n<p>No. Residues of PVC release hydrogen chloride, which catalyses POM decomposition violently. POM needs a dedicated barrel or a very thorough purge with PE followed by purging compound.<\/p>\n<h3>Why does POM need a hot mould?<\/h3>\n<p>Because it is highly crystalline and the crystallinity \u2014 and therefore the strength, wear resistance and dimensional stability \u2014 depends on how slowly it cools. At 60 \u00b0C the part is under-crystallised; at 100 \u00b0C it is fully crystalline. The longer cycle is the price of the properties you specified the material for.<\/p>\n<h3>Does POM need drying?<\/h3>\n<p>Usually not if it comes in sealed bags. POM absorbs only 0.2 % in 24 hours and that level does not cause defects. If bags have been open in a humid plant for weeks, dry at 80 \u2013 90 \u00b0C for 2 hours.<\/p>\n<h3>POM-H or POM-C?<\/h3>\n<p>POM-H for maximum stiffness, strength and creep resistance \u2014 gears, precision parts. POM-C for thermal and chemical stability, hot water, food contact, and any job where the machine or tool might run hot. POM-C is the safer default.<\/p>\n<h3>Can POM be painted or bonded?<\/h3>\n<p>Poorly. POM has very low surface energy and almost nothing sticks without a specific pre-treatment. Plasma treatment works; flame treatment is marginal; most adhesives fail. Design for mechanical assembly or ultrasonic welding instead.<\/p>\n<h3>What causes black specks in POM parts?<\/h3>\n<p>Carbonised material \u2014 usually in a hot-runner dead spot or behind the screw check ring. POM is unforgiving here because the decomposition products are dark and corrosive. Strip and clean, then check the hot-runner tips for a step or a cavity where material can sit.<\/p>\n<\/div>\n<div class=\"cta\">\n<p class=\"cta-h\">Moulding a POM gear or bearing and need the tolerance to hold?<\/p>\n<p>Send the drawing and the service conditions. We will tell you the right homopolymer or copolymer, the mould temperature the part needs, and whether the tool should be cut for 2 % or 2.5 % shrinkage.<\/p>\n<p><a class=\"btn\" href=\"https:\/\/uplastech.com\/tr\/pom-acetal-injection-molding\/\">Talk to a POM moulding engineer<\/a>\n<\/div>\n<div class=\"rel\">\n<h4>Continue reading<\/h4>\n<ul>\n<li><a href=\"https:\/\/uplastech.com\/tr\/nylon-pa-injection-molding-guide\/\">Nylon (PA6 \/ PA66) Injection Molding: Moisture, Conditioning and Dimensions<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/tr\/pbt-pet-injection-molding-guide\/\">PBT and PET Injection Molding: Glass Fibre, Drying and Warpage<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/tr\/injection-molding-tolerances\/\">Injection Molding Tolerances: What Is Actually Achievable<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/tr\/injection-molding-defects\/\">Injection Molding Defects: 12 Problems, Causes and Fixes<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/tr\/injection-molding-material-selection\/\">Injection Molding Material Selection: 14 Plastics Compared<\/a><\/li>\n<\/ul>\n<\/div>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>Acetal moulding: the 220 \u00b0C ceiling and why it matters, homopolymer versus copolymer, mould temperature and crystallinity, stainless tooling and machinability.<\/p>","protected":false},"author":1,"featured_media":7601,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_seopress_titles_title":"POM Acetal Injection Molding: Stability & Tooling","_seopress_titles_desc":"Acetal moulding: the 220 \u00b0C ceiling, homopolymer versus copolymer, mould temperature and crystallinity, stainless tooling and machinability.","_seopress_robots_index":"1","_seopress_robots_follow":"1","_seopress_robots_imageindex":"","_seopress_robots_snippet":"","_seopress_robots_primary_cat":"35","_seopress_robots_breadcrumbs":"","_seopress_robots_freeze_modified_date":"","_seopress_robots_custom_modified_date":"","_seopress_robots_canonical":"https:\/\/uplastech.com\/pom-acetal-injection-molding-guide\/","_seopress_social_fb_title":"","_seopress_social_fb_desc":"","_seopress_social_fb_img":"","_seopress_social_fb_img_attachment_id":0,"_seopress_social_fb_img_width":0,"_seopress_social_fb_img_height":0,"_seopress_social_twitter_title":"","_seopress_social_twitter_desc":"","_seopress_social_twitter_img":"","_seopress_social_twitter_img_attachment_id":0,"_seopress_social_twitter_img_width":0,"_seopress_social_twitter_img_height":0,"_seopress_redirections_value":"","_seopress_redirections_enabled":"","_seopress_redirections_enabled_regex":"","_seopress_redirections_logged_status":"","_seopress_redirections_param":"","_seopress_redirections_type":0,"_seopress_analysis_target_kw":"pom acetal injection molding","footnotes":""},"categories":[35],"tags":[],"class_list":["post-7609","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-injection-molding"],"meta_box":{"post-to-quiz_to":[]},"_links":{"self":[{"href":"https:\/\/uplastech.com\/tr\/wp-json\/wp\/v2\/posts\/7609","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/uplastech.com\/tr\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/uplastech.com\/tr\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/uplastech.com\/tr\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/uplastech.com\/tr\/wp-json\/wp\/v2\/comments?post=7609"}],"version-history":[{"count":0,"href":"https:\/\/uplastech.com\/tr\/wp-json\/wp\/v2\/posts\/7609\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/uplastech.com\/tr\/wp-json\/wp\/v2\/media\/7601"}],"wp:attachment":[{"href":"https:\/\/uplastech.com\/tr\/wp-json\/wp\/v2\/media?parent=7609"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/uplastech.com\/tr\/wp-json\/wp\/v2\/categories?post=7609"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/uplastech.com\/tr\/wp-json\/wp\/v2\/tags?post=7609"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}